In a sputtering deposition process ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field causing atoms of the target material to be ejected from the cathode surface. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate.
Sputter coating is a widely used technique for depositing a thin film of material on a substrate. Sputtering is the physical ejection of material from a target as a result of gas ion bombardment of the target. In one form of this technique, known as DC sputtering, positive ions from a plasma discharge formed between an anode and a target cathode are attracted to and strike the target cathode, dislodging atoms from the target surface of the cathode thereby providing sputtering atoms. Some of the dislodged atoms impinge on the surface of the substrate and form a coating. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material.
The sputtered material is also deposited on any other surface exposed to the sputtered atoms. It is recognized in the prior art that if the coating is an electrically insulating material, such as a metal oxide, the build up of the material on other parts of the sputtering apparatus can cause problems. In particular, the build up of an insulating coating on the anode interferes with the ability of the anode to remove electrons from the plasma, as required to maintain the plasma's charge balance. This destabilizes the plasma and interferes with deposition control. As a result, it is common to use a different sputtering technique, for example RF sputtering, to deposit layers of insulating materials. However, RF sputtering is a less efficient, less controllable, slower and more expensive process than DC sputtering.
In operation, when the argon is admitted into a coating chamber, the DC voltage applied between the target cathode and the anode ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target. The ions strike the target with a substantial energy and cause target atoms or atomic clusters to be sputtered from the target. Some of the target particles strike and deposit on the wafer or substrate material to be coated, thereby forming a film.
In an endeavor to attain increased deposition rates and lower operating pressures, magnetically enhanced targets have been used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field causes the electrons to travel in a closed loop, commonly referred to as a “race track”, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability thereby leading to a much higher sputtering rate than that obtained without the use of magnetic confinement. Furthermore, the sputtering process can be accomplished at a much lower gas pressure.
As mentioned heretofore, in DC reactive sputtering, a reactant gas forms a compound with the material which is sputtered from the target plate. When the target plate is silicon, and the reactive gas is oxygen, silicon dioxide is formed on the surface of the substrate. However, because silicon dioxide is a good insulator, a film thick enough to cause arcing is rapidly formed in areas outside of the race track, e.g. on electrically grounded dark space shields. Silicon dioxide is known to be one of the most difficult dielectric films to deposit by magnetron reactive sputtering because of this characteristic. The arcing associated with silicon dioxide has prevented planar magnetron reactive sputtering from being efficiently utilized to deposit high quality silicon dioxide films. One aspect of this invention provides a coated cathode having its sides and bottom surface coated with a dielectric to lessen or obviate arcing.
In operation, due to the accumulation of dielectric material in various parts of the coating chamber, it has been necessary to clean the system on a regular basis. Indeed, when coating silicon dioxide or silicon nitride by reactive sputtering, typical systems can only operate continuously for relatively short periods of time.
Finally, another limitation to the utility of planar and cylindrical magnetrons in either reactive or non-reactive sputtering is that films deposited by sputtering have not achieved the degree of uniformity or repeatability required for many high precision applications.
There have been many attempts to ameliorate these unwanted effects of magnetron sputtering. For example “bottle-brush” anodes have been proposed and are described in U.S. Pat. No. 5,683,558 in the name of Sieck et al, issued Nov. 4, 1997. This kind of anode advantageously provides a large surface but, depending on its position relative to the target, it becomes non-uniformly coated over time and causes the anode to move to other surfaces in the deposition system. Additionally, the distance between the brush needles is very close and often leads to an increased anode voltage, especially at low pressure.
Anode plates and ring designs have been described by F. Howard Gillery et al., PPG Industries Inc., in U.S. Pat. No. 4,478,702, entitled “Anode for magnetic sputtering apparatus”, and in U.S. Pat. No. 4,744,880, entitled “Anode for magnetic Sputtering of gradient films”, and by P. Sieck in “Effect of Anode Location on Deposition Profiles for Long Rotatable Magnetrons”, SVC, 37th Annual Technical Conf. Proceed., 233, (1994).
Anode plates and ring designs described by J. R. Doyle, et al., in J.Vac. Sci. Technol. A12, 886 (1994) are the most widely used design for anodes. Typically the anode is in close proximity to the cathode to enable a sufficient coupling of the anode-to-cathode plasma. Most often the gas inlet is close to the target surface to increase the target pressure locally. Most of the time the anode surface is also close to that location which increases the plasma coupling and reduces the anode voltage. Unfortunately these types of anodes can't be positioned too far behind the cathode, because the electrons have to cross magnetic field lines on route to the anode which adds a high resistance and increases the anode voltage. On the other hand, having the anode close to the cathode surface increases the anode's susceptibility to being coated with sputtered material thereby making the anode unstable.
It is known to position dispose the anode in close proximity to other plasma sources out of the direct line of sight of the cathode. This approach works for relatively thin coatings, for example coatings of less than 5 μm, but for thicker films the anode becomes coated as well due to gas scattering. This makes it necessary to routinely exchange the anodes, which increases the cycle time and adds to the cost.
One disadvantage to the aforementioned approaches is that the size of the anodes has to be relatively large to work at a reasonably low voltage. The large size leads to an uneven contamination of the anode surface and to a change in sputter distribution. Furthermore, a large anode has to be accommodated within the coating chamber where space is typically lacking.
A small filament-like anode is another form of prior art anode. This anode requires relatively high voltages for example, greater than 70 V, which typically leads to undesirable sputtering of surfaces at or near the anode. The anode has to be positioned very close to the cathode for sufficient coupling. Additionally, major changes to the magnetron generally have to be made by way of shunting the magnetic fields close to the anode.
Dual magnetron AC sputtering has been proposed by S. Schiller, K. Goedicke, V. Kirchhoff, T. Kopte in “Pulsed Technology—a new era of magnetron sputtering”, 38th Annual Technical Conference of SVC, (1995).
This approach inherently solves the moving anode and disappearing anode problems of some of the aforementioned prior art anodes, but the sputtering rates are usually lower and AC sputtering needs a higher pressure to run at decent cathode voltages <900 V. This increases the gas scatter and thus the defect growth. But even at ‘low’ average voltage the peak voltage in this setup is very high and often greater than 1000 V and leads to an increased compressive stress in the coating. The high voltage is caused by igniting the plasma every half-cycle at each cathode.
The very recent approach of Dual anode magnetron sputtering to solve the anode problem uses a dual anode AC configuration. Preliminary tests showed that the anodes have to be highly coupled into the cathode plasma. Thus, they have had to be positioned very close to the cathode. Because the anode reaches very high negative voltages, this causes sputtering from the anode during the cleaning cycle. In a paper entitled “Redundant Anode Sputtering: A Novel Approach to the Disappearing Anode Problem”, published on the internet at the following website: http://www advanced-energy.com/upload/white2.pdf, several disadvantages of Dual cathode AC sputtering are mentioned.
Since the anode is generally close to the target, it is exposed to coating material. In practice, in many prior art systems, anodes have to be exchanged or cleaned at frequent, regular intervals. Even when the anode is out of a direct line of sight of the direct material flux, the anode becomes coated due to gas scattering of coating material.
It is an object of this invention to provide an anode that is extremely well shielded from coating material. The provision of such an anode leads to a more stable sputtering process, especially for very thick coatings, and reduces or eliminates the maintenance of the anode. This reduces the cycle time and labor costs in coating substrates.
It is a further object of this invention to provide an anode which can be pressurized and which requires a lower voltage than many prior art anodes. Although the anode can be pressurized, it operates within or communicates with a chamber under vacuum.
It is a further object of this invention to provide an anode wherein little or no arcing at or near the anode occurs.
It is yet a further object of this invention to provide a preferred cathode for use with the anode of this invention.